1. Field of the Invention
The present invention relates to an automatic transmission for a motor vehicle and, more particularly, to a transmission control apparatus capable of, during progress of a shift operation to one speed, deciding and initiating a shift to a different speed (overlap shift).
2. Description of the Related Art
One type of known automatic transmission includes a main speed shift mechanism providing three forward speeds and one reverse speed and an auxiliary speed shift mechanism providing three speeds. The combination achieves multiple speeds, for example, five forward speeds and one reverse speed. Such an automatic transmission may make a shift decision dictating a shift involving one of the two shift mechanisms during a shift operation of the other shift mechanism. For example, while the automatic transmission is performing the 5th-to-4th speed shift (a first shift) operation by releasing a predetermined frictional engagement element (C2 clutch) of the main speed shift mechanism (and simultaneously engaging a one-way clutch F1), a decision may be made to shift to the 3rd speed (a second shift) by changing over the engagement of two frictional engagement elements (by releasing a C3 clutch and engaging a B4 brake) of the auxiliary speed shift mechanism.
One such conventional automatic transmission, disclosed in examined Japanese patent application publication No. HEI-5-50621, determines that the operation of shifting to a given speed, responsive to a first shift instruction, has been completed on the basis of a determination that the input and output rotational speed ratio, at the speed dictated by the first shift instruction, is approximately 1.0, over at least a predetermined length of time. If a second shift instruction, different from the first speed shift instruction, generated during the period between the output of the first shift instruction and the completion of the shift dictated by the first instruction, the automatic transmission prevents a shift operation responsive to the second shift instruction. That is, if during a first shift operation a decision for a second shift operation is made, the automatic transmission inhibits the second shift operation until the first shift operation is completed. This shift operation inhibition prevents deviation of the timing of engagement changeover of frictional engagement elements, thus preventing a rapid engine speed increase or an abnormal engine speed reduction. However, because the second shift is inhibited until the first shift operation is completed, this conventional system has problems in that the shift time between the start of the first shift and the completion of the second shift is prolonged.